Photothermographic imaging systems are those imaging materials which, upon first being exposed to light in an imagewise fashion, produce an image when subsequently heated. The exposure to light or other radiation photoactivates or photodeactivates a component in the imageable element and subsequent heating causes an image forming reaction to differentially occur in exposed and unexposed regions.
A variety of different types of photothermographic technologies exist in the marketplace. Thermal diazonium systems such as those disclosed in U.S. Pat. Nos. 4,230,789; 4,168,171 and 3,754,916 comprise an acid-stabilized light-sensitive diazonium salt, a compound that couples with diazonium salts (known as an azo-coupling compound), and a neutralizing compound which becomes basic, releases a base by decomposition, or is basic and migrates to the acid-stabilized diazonium salt upon being heated. These components are in a binder system coated onto a support base.
Another well known photothermographic imaging system is described in U.S. Pat. Nos. 3,457,075; 3,839,049 and 3,994,732. These imageable systems comprise a silver source material (usually an organic silver salt, a silver salt of an organic long chain fatty carboxylic acid, or a complexed silver salt), silver halide in catalytic proximity to the silver source material, a reducing agent for silver ion, and a binder.
Other photothermographic imaging systems comprising leuco dye oxidation systems and dye-bleach systems such as those described in U.S. Patent Application Serial Nos. (Winslow, Winslow-Gatzke, Gatzke case) are also useful systems.
Each of these systems are used either by first exposing the element to light and then having the entire element heated (e.g., on a heated drum roll, in an inert oil bath, or by exposure to infrared radiation) or by heating and exposing the element contemporaneously. All of these forms of heating tend to be energy inefficient and may cause unequal development of the image because of unequal heating. To overcome some of these difficulties, a few recent products having opaque support layers have been provided with a conductive layer such as vapor deposited metal or carbon black-filled polymeric resin. This conductive layer, or more accurately resistive layer, allows the element to be heated by the application of a voltage across the layer. The voltage must be sufficient to generate heat in the resistive layer. The heat generated can then be sufficient to thermally develop an image on an exposed photothermographic element. The resistive layer is not particularly aesthetically pleasing when viewed from the back and cannot be used with a transparent substrate, particularly when the final image is to be projected, because the resistive layer is often opaque. Furthermore, the resistive layer, if a thin (e.g., vapor deposited) metal layer, is readily subject to damage and discontinuities which would appear as defects in the final image.